Toner pump

ABSTRACT

A controlled standing wave of developer is formed above a pair of spaced baffle plates by a pair of axially elongated counterrotating gears supported below the baffle plates in a tank of developer. An evaporation sealer is pivotably positioned to sealably fit over the pair of baffle plates when the spur gears stop rotating to prevent evaporation of the developer.

United States Patent Schrempp et a1.

1541 TONER PUMP [72] Inventors: Ernst Schrempp, Norwalk; Christian A. Beck, Ridgefield, both of Conn.

[7 3] Assignee: Pitney-Bowes, lnc., Stamford, Conn.

[22] Filed: May 11, 1971 [21] Appl. No.: 142,295

[52] US. Cl ..118/429, 228/37 [51] Int. Cl ..B05c 3/12 [58] Field of Search ..118/410, 429, 300, 121,114;

[56] References Cited UNITED STATES PATENTS 2,529,699 ll/l950 Lach ..118/123 2,759,850 8/1956 Knopf ..117/114 A 3,092,059 6/1963 Tesch, Jr ..228/37 [451 Dec. 5, 1972 3,119,363 Rieben ..228/37 3,190,527 6/1965 Tardoskegyi ..228/37 3,198,414 8/1965 Tardoskegyi ..228/37 3,256,855 6/1966 Oliphant .1 18/DlG. 23

3,430,332 3/1969 Koller ..228/37 X 3,604,611 9/1971 Lamberty ..228/37 X Primary Examiner-Morris Kaplan Attorney-William D. Soltow, Jr., Albert W. Scribner, Martin D. Wittstein and Louis A. Tirelli [57] ABSTRACT A controlled standing wave of developer is formed above a pair of spaced baffle plates by a pair of axially elongated counter-rotating gears supported below the baffle plates in a tank of developer. An evaporation sealer is pivotably positioned to sealably fit over the pair of baffle plates when the spur gears stop rotating to prevent evaporation of the developer.

11 Claims, 7 Drawing Figures PATENTED DEC 5 I972 SHEET 1 0F 2 FIGS.

SP RD 0 N R H V 6 m5 T s N R E CHRISTIAN A. BECK ATTORNEY 'PATENTED EB 5|972 3.704.686

sum 2 0F 2 PRIOR ART INVENTORS |:|(5 6 F161 7 ERNST SCHREMPP CHRlSTIAN A. BECK BY I 9 4 ATTORNEY TONER PUMP BACKGROUND OF THE INVENTION This invention relates generally to development of latent electrostatic images and in particular it relates to development of the latent image on sheets or the like by applying liquid developer onto one face of the sheets especially on the areas containing the latent image.

In the process of xerography, it is usual at one or another point to subject a latent electrostatic image to development. This step commonly involves contacting the latent image with a suitable pigment, usually referred to as toner. Such toner may be brought into contact with the latent image in a number of different ways. The toner may, for example, be dusted as a dry powder directly upon the latent image. In another technique, the toner particles may be first dispersed throughout a liquid medium and quantities of the pigment-carrying liquid are then brought into contact with the latent image. This latter type of process is known as liquid development.

Liquid development has a number of advantages over dry techniques. In most dry techniques, for example, the toner particles are electrostatically bound to a carrier. As a result, a minimum of force must be overcome before it is even possible for the toner particles to be attracted to the electrostatic image. This minimum value, which is called a threshold value is not present in the case of liquid developers where the particles, of course, are freely suspended. Another obvious advantage of liquid developers arises by virtue of the fact that very minute toner particles can be readily suspended in the liquid medium while they are not so readily handled and distributed in dry form.

As a result of these and other advantages, it is theoretically possible to obtain higher image resolutions with liquid developers than can be accomplished by dry methods. In practice, however, liquid developers have not proven so ideal.

Firstly, it has been found that when a sheet is immersed in a tank of developer, both sides of the sheet are wetted so that either large volumes of heated air must be passed over the wetted sheet for drying or the time period for drying must be sufficiently long to insure that a dry copy will be dispensed from the machine. Further, the kerosene-like odor attending the evaporation of relatively large quantities of developer on the sheet is unacceptable in an office environment.

Secondly, the immersed sheet carries toner out of the tank on both faces, whereas toner is only required on the one face having the latent image thereon. Thus, there is an inefficient use of the toner which is applied to the sheet in the immersion system.

To avoid this rapid depletion of toner, one side developer contacting devices have been suggested such as those which use cascading surfaces to develop a standing wave, rollers and sprays.

It has been found that when a xerographic plate or other latent image bearing surface is brought into contact with a stationary volume of liquid developer, portions of the developer in immediate contact with the latent image areas tend to become depleted of toner particles thereby depriving more weakly charged areas of sufficient development. In an attempt to overcome this depletion, turbulence or general motion is introduced into the developer, but this general motion brings with it a tendency to mechanically and triboelectrically destroy the very delicate latent image. Further, if rollers operated to produce a standing wave are speeded up to bring additional toner into the standing wave a bad wave form is produced as well as other possible uncontrollable factors and uniform development of the latent image is impeded.

One apparatus which has been taught to overcome this depletion of toner is found in U.S. Pat. No. 3,334,613 by Young, entitled Xerographic Development Apparatus. Youngs device requires pumps for air pressure and developer circulation as well as a developing area open to the atmosphere having alternating ridges and troughs. Youngs device is expensive to manufacture and operate which in an industry where competition is keen, can be detrimental to sales. Further, Youngs device provides a development area which will be covered with a surface film which film increased with thickness in proportion to the number of periods of use. This is due to evaporation of developer in the troughs when the equipment is switched off during periods of non-use. As stated above, the odor present from the developer evaporating is a very strong kerosene-like odor which is unacceptable in'an office or other similar environment.

A spray would appear to be a good compromise. However, sprays with their necessarily small nozzles require very fine particle size toners which cannot provide a high image resolution. Also, the spray provides a decreasing concentration of toner from the center to the peripheral edges and thus an accompanying uneven development of the latent image. Furthermore even with the use of a very fine particle size toner the nozzle still becomes clogged, disabling the machine.

Apparatus using contacting rollers to apply the developer onto the face of the sheet dispense too thin a film of developer onto the sheet and thus the image resolution is extremely low. Even if the amount of developer could be increased with apparatus using a roller, the application of the developer onto the sheet produces anv uneven image. This is because lateral boundaries of developer contacting the sheet are irregular lines producing uneven longitudinal contact of the liquid developer with the surface of the sheet. Thus, certain longitudinal sections of the sheet are in contact with the developer longer than other sections and a striated visible image is created.

Smooth surface rollers which produce a standing wave of developer such as those disclosed in U.S. Pat. No. 3,203,395 by Liller entitled Apparatus for Developing Electrostatic Images and in U.S. Pat. No. 3,256,855 by Oliphant entitled Machine for Applying Liquids, produce an uncontrolled standing wave to wet only one face of the sheet. However, because of the smoothness of the roller this apparatus is limited for use only with fine particle toner since larger particles are unable to adhere to the smooth surface roller. As stated above, fine particle toner will not produce the contrast available with courser particle toner. Producing the rollers with a napped surface will only bring up larger particles of toner to the standing wave for a limited time until the pores or depressed areas of the rollers are filled with toner and function as smooth surface rollers. Smooth surface rollers also have the disadvantage of uneven development of the latent image because of lack of control of the wave geometry which permits an uneven contact of developer and sheet surface.

SUMMARY OF THE INVENTION It is accordingly, an object of the present invention to provide a trough of developer having a controlled waveform.

It is another object of the present invention to provide a means for agitating developer sufficiently to obtain a constant distribution of toner throughout the portion of the developer in contact with the sheet being developed.

It is a further object of the present invention to pro vide an apparatus which will controllably wet only one face of a sheet being developed. v a

It is still a further object of the present invention to provide apparatus whereby turbulent agitation may be introduced into liquid developer in order to maintain suspension of toner particles, and yet such agitation is so introduced that the sensitive latent electrostatic image is not thereby damaged.

It is still another object of the present invention to provide apparatus which makes possible a controlled, gentle and non-abrading multiple contact of large volumes of circulating liquid developer with the latent electrostatic image, so that the latent image is not thereby destroying or otherwise disturbed.

It is an added object of the present invention to provide a means for raising quantities of liquid developer from the level of the liquid in the tank into the baffle means so that the baffle means is always filled with circulating liquid developer which makes possible multiple contact thereof with the latent electrostatic image.

In accordance therewith, the present invention accomplishes these objects and advantages by providing an apparatus for wet development of latent electrostatic images which comprise a tank having a base and upstanding walls to contain a quantity of liquid developer, fluid displacement means for raising the developer above the level of the liquid in the tank, actuating means for driving the fluid displacement means at a speed sufficient to create a standing wave of constantly replenished developer at a location generally above the fluid displacement means, and baffle means for receiving developer from thefiuid displacement means and defining an area of containment for the standing wave and for stabilizing the surface characteristics of the standing wave within the area of containment whereby the crest of the wave presents uniform intimate contact with the surface of a sheet of copy paper passing thereover.

BRIEF DESCRIPTION OF THE DRAWING The present invention will be more readily apparent from an understanding of the following detailed description of one embodiment of the present invention when considered in conjunction with the accompanying drawings in which like reference numerals refer to like elements in the various figures and m which:

FIG. 1 is a perspective view of one form of the invention, broken away in part to show the gears;

FIG. 2 is a longitudinal cross section taken along the line 2-2 of FIG. 1, showing the apparatus in operation;

FIG. 3 is a view similar to FIG. 2 but showing the apparatus in a non-operating condition;

FIG. 4 is a view also similar to FIG. 2 of a modified embodiment of the apparatus shown in operation;

FIG. 5 is a dimensioned sectional elevation of the baffles and gears taken along the lines 22 of FIG. 1 and shown on an enlarged scale; and

FIGS. 6 and 7 are views of the face of the sheet being contacted with developer at an illustrative midsection as the sheet is drawn over the development area of the inventive apparatus.

DETAILED DESCRIPTION Referring to the drawings, there is seen a toner circulation device which comprises a tank 4 having a base 6 and upstanding walls 8. The tank 4 containsa supply of liquid developer 9 having a normal liquid level disposed well below the top of the tank 4 at a location inaccessible to copy paper passing over the tank 4.

In order to raise developer 9 from the supply in the tank 4 to a level thereabove at which the developer 9 becomes accessible to copy paper passing over the tank 4, there is provided a fluid displacement means. The fluid displacement means includes rotating means having a contoured peripheral surface such as a pair of axially parallel counterrotating gears 10 and 12, extending radially below the liquid level and journailed in opposite ones of the walls 8. In operation, the gears 10 and 12 are partially submerged for rotation in the developer 9.

By a contoured peripheral surface is meant that the rotating means have a peripheral configuration suitable to lift a liquid such as developer from the liquid level in the tank 4 to a level above the tank 4. This configuration is other than the smooth or napped cylindrical surface of a roller. The root diameters of the gears 10 and 12 can be radially spaced from each other between a minimum distance (as shown in FIG. 4) so that the teeth engage each other in conventional meshing engagement up to a maximum distance (as shown in FIG. 2) wherein the gears 10 and 12 are spaced to cooperate to create a suspended pool 13 in the area above and between the gears 10 and 12. Either or both of gears 10 and 12 can be spur gears or in a preferred mode the teeth 14 on the spur gears 10 and 12 can have a concave surface formed on the intended upstream face as the gears 10 and 12 are intended to rotate, i.e., either (I) the spur gear teeth are initially extruded with a concave portion on the intended upstream face or (2) the intended upstream face of each tooth on a conventional spur gear is machined to a concave shape so that each tooth will carry liquid therein for at least a portion of a complete revolution.

The tank 4 is partially covered by a first and second splash plate 16 and 18 respectively, each extending from an opposite one of the walls 8 radially distant from the gears 10 and 12. First and second baffles 20 and 22, respectively extend down toward the tank 4 from the respective inner edges 24 and 26 of the splash plates 16 and 18. The baffles 20 and 22 each form an angle A, which is within an operable range of about 25 to 35 (preferably at an angle of 30) with the plane of the splash plates 16 and 18.

The baffles 20 and 22 are spaced from each other to form a slot 28 for receiving developer from the liquid displacement means therebetween. The developer is raised by the concave portion of the teeth 14 to above the liquid level in the tank 4 through the slot 28 and into the pool 13 of developer supported in part by the baffles 20 and 22 and in part by those teeth 14 contacting the bottom of the pool 13.

To be effective, the length of the first baffle 20 must be within an operable range of about to percent of the pitch diameter (i.e. the diameter of the pitch circle) of the gears 10 and 12. For the second baffle 22 to be effective, the length thereof must be within an operable range of about 25 to 35 percent of the pitch diameter of the gears 10 and 12. The gap W of the slot 28 is the perpendicular distance between verticals dropped from the spaced edges 24 and 26 of the first and second baffles and 22 respectively, and to be effective must be within an operable range of about 28 to 32 percent of the pitch diameter of the gears 10 and 12.

The baffles 20 and 22 define an area of containment or a trough 40 for a standing wave and stabilize the surface characteristics of the standing wave 42 within the area of containment.

The first baffle 20 is shorter than the second baffle 22 to allow developer being raised by the teeth 14 to go through the slot 28 as gear 10 rotates clockwise and gear 12 rotates counterclockwise. Actuating means are provided for counterrotating the gears 10 and 12. Specifically, in the embodiment shown in FIG. 1, the gears 10 and 12 are respectively connected axially to a single groove pulley 30 and to a double groove pulley 32. A motor 34 is mounted on the reservoir wall 8 and is conventionally connected to a drive pulley 36. A drive belt 28 is fitted about the outer groove of the pulley 32 and the drive pulley 36. A driven belt 31 is looped in a figure 8 about the pulley 30 and the inner groove of the pulley 32, in order to drive the pulleys 30 and 32 and their connected gears 10 and 12 clockwise and counterclockwise, respectively. The pulleys in turn conventionally drive the gears 10 and 12 in counterrotating directions.

In operation, the motor 34 is activated to counterrotate the gears 10 and 12 through a conventional linkage such as, for example, the illustrated drive pulley 36, belt 38, and the pulleys 30 and 32. The developer 9 in the tank 4 is agitated by the counter rotating gears 10 and 12 so that toner in the developer is homogenously mixed throughout the developer while developer is raised through the slot 28 to build up the pool 13. In a very short time, at a predetermined speed of rotation, a steady state of loss and gain of developer in the pool 13 is attained. The surface of the developer in the pool 13 takes on a smooth glass-like appearance with a narrow controlled band or stabilized standing wave 42 resembling a smooth swell appearing at approximately the midsection of the pool 13.

It should be understood that with the developer in the pool 13 in the steady state condition last described, there is a turbulence below the surface of the pool 13 continually moving toner into the stabilized standing wave 42 during conventional developing. Although the theory for this is not quite understood, it is speculated that the first and second baffles 20 and 22 respectively, stabilize the surface characteristics of the stabilized standing wave 42 while directing the developer displaced from the tank 4 into the pool 13 back on itself below the surface thereof to absorb the kinetic energy in the turbulent portion of the developer.

Development of the latent image on a sheet 44 is accomplished by passing the sheet 44 through a first pair of rollers 46 with the latent image thereon face down to contact the smooth swell 42 as it is moved thereover. A second pair of rollers 48 are conventionally positioned to forward the sheet 44 on to the next station.

The contact segment 50 of the smooth swell is shown in FIG. 7. It has been observed that as the face of the longitudinally moving sheet 44 makes contact with the smooth swell 42, the contact segment 50 has a rectangular border and development of the latent image is uniform throughout since the time that developer is in contact with the sheet is uniform across the width of the sheet.

This is in contrast to prior art development of copy which is demonstrated in FIG. 6 wherein the lateral contact borders above and below the standing wave on the sheet 44 are uneven. Therefore each longitudinal segment has a separate image resolution since each segment is given a different intensity of toning as a result of the different periods of time that developer is in contact with the sheet 44 as indicated by lines a, b and c. Thus the developed image on the sheet 44 is striated i.e., it has a streaked appearance, the darker streaks being where the developer was in contact with the latent image for a longer period of time.

To minimize evaporation of the developer when the device is not in use and simultaneously to avoid the presence of the rather disagreeable kerosene-like odor which emanates from the developer in the reservoir, FIGS. 1 to 4 illustrate a tank sealing means 51 having an arm 52 conventionally non-rotatably mounted at its free end about a pin 54, and having a cover or closure portion 55. The tank sealing means 51 minimizes the evaporation of developer 9 from the tank 4 when the counterrotating gears 10 and 12 are no longer being rotated after the motor 34 is turned off.

The tank sealing means 51 is raised to above the plane of the path of the sheet 44 when the counterrotating gears 10 and 12 are initially rotated, by a conventional motor control and timing circuit (not shown) which is conventionally activated when the motor 34 is activated and operated only long enough to bring the tank sealing means 51 to its raised position.

When the motor 34 is turned off the tank sealing means 51 is lowered to a closed position by the motor control and timing circuit until the closure portion 55 rests in the trough 40. When the motor 34 is initially turned off, a conventional time delay mechanism such as a time delay relay (not shown) is activated to delay starting the control motor which lowers the tank sealing means 51 so that the closure portion 55 does not enter the trough 40 until the developer has drained therefrom. The developer begins to drain from the trough 40 as soon as the motor 34 and the counterrotating gears 10 and 12 stop. The tank sealing means 51 by minimizing evaporation of developer also avoids the attendant accumulation of dirt and residue on the first and second baffles 20 and 22 respectively.

The tank sealing means 51 can be conventionally fabricated as a composite or unitary piece from metal or a plastic which is not soluble in the developer. The closure portion 55 fits tightly in the trough 40 and sits on top of the first and second baffles 20 and 22 respectively, in a cooperative sealing association.

it is thus apparent from the foregoing that there has been provided an apparatus for creating a smooth swell in a reservoir of the present invention. The teeth on the gears can have any shape which will carry liquid on the face of the teeth as the gears are rotated and the drive for the gears can include a belt drive, gear drive, friction drive, chain drive or any other conventional drive means.-

We claim:

1. An apparatus for wet development of latent electro-static images, comprising;

a. a tank having a base and upstanding walls to contain a quantity of liquid developer;

b. fluid displacement means for raising said developer above the level of the liquid in said tank;

c. actuating means for driving said fluid displacement means at a speed sufficient to create a standing wave of constantly replenished developer at a location generally above said fluid displacement means; i

. a pair of opposed baffle means disposed above said level of the liquid and extending substantially across the tank and defining therebetween a slot for receiving developer from said fluid displacement means and defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment, whereby the crest of said standing wave presents uniform intimate contact with the surface of a sheet of copy paper passing thereover; and

e. a tank sealing means adapted to automatically seal said slot to thereby completely cover said tank when the fluid displacement means is shut down.

2. An apparatus as defined by claim 9, wherein said fluid displacement means includes rotating means hav' ing a contoured peripheral surface.

3. An apparatus as defined by claim 2, wherein said rotating means includes a pair of axially parallel counterrotating gears journalled in opposite ones of said upstanding walls and extending radially below said liquid level, said gears being radially spaced from each other no closer than meshing engagement and no greater than would allow said gears to cooperatively create a suspended pool in the area thereabove and therebetween.

4. An apparatus as defined by claim 3, wherein said gears are axially elongate spur gears.

5. An apparatus as defined by claim 3, wherein the leading faces of the teeth on said gears form a concave surface, whereby said teeth raise larger amounts of developer from said liquid level to said receiving means.

6. An apparatus as defined by claim 1, wherein said baffle means includes a first and second splash plate having first and second baffles extending respectively from inner edges thereof and spaced from each other to form a slot therebetween, said first and second baffles forming an angle with the plane of said first and second splash plates, respectively, of between about 25 and 35, whereby said developer is raised by said fluid displacement means through said slot and into said baffle means.

7. An apparatus as defined by claim 6, wherein the gap width of said slot is between about 28 and 32 percent of the pitch diameter of said gears.

8. An apparatus as defined in claim 6, where the length of said first baffle is between about 10 and 15 percent of the pitch diameter of said gears and the length of said second baffle is between about 25 and 35 percent of said pitch diameter.

9. An apparatus for wet development of latent electrostatic images, comprising:

a. a tank having a bottom wall and upstanding walls to contain a liquid developer;

b. -fluid displacement means for raising said developer above the level of the liquid in said tank;

c. actuating means for driving said fluid displacement means at a speed sufficient to create a standing wave of constantly replenished developer at a lo-. cation generally above said rotating means;

. a rectangularly shaped first baffle suitably supported over said fluid displacement means and pitched therefrom toward said bottom wall; and

e. a rectangularly shaped second baffle suitably supported over said fluid displacement means and pitched therefrom toward said bottom wall at an equal and opposite angle to the angle at which said first baffle is pitched, said second baffle being longer than said first baffle and being spaced from said first baffle to form a slot which is cooperatively associated with said fluid displacement means to receive said developer raised thereby, said first and second baffles defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment,

whereby said fluid displacement means raises developer from said liquid level through said slot and the crest of said standing wave presents uniform intimate contact with the surface of a sheet of copy paper passing thereover.

10. An apparatus for wet development of latent electrostatic images, comprising; 7

a. a tank having a base and upstanding walls to contain a quantity of liquid developer;

b. fluid displacement means for raising said developer above the level of the liquid in said tank;

c. actuating means for driving said fluid displacement means to create a standing wave of constantly replenished developer at a location generally above said fluid displacement means;

d. baffle means for receiving developer from said fluid displacement means and defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment; and

. tank sealing means for minimizing the evaporation of developer from said tank when said fluid displacement means are not in motion, said tank sealing means including;

1. a cover cooperatively associated with said baffle means; and

II. means for raising said cover above said baffle means when said fluid displacement means are being driven and for lowering said cover onto said baffle means when said fluid displacement means are not being driven;

whereby said cover is lowered on top of said baffle means when said fluid displacement means are not being driven and said cover is raised when said fluid displacement means are being driven.

11. An apparatus as defined by claim 10, which additionally includes a time delay control means for maintaining said tank sealing means in a raised position for a 

1. An apparatus for wet development of latent electro-static images, comprising; a. a tank having a base and upstanding walls to contain a quantity of liquid developer; b. fluid displacement means for raising said developer above the level of the liquid in said tank; c. actuating means for driving said fluid displacement means at a speed sufficient to create a standing wave of constantly replenished developer at a location generally above said fluid displacement means; d. a pair of opposed baffle means disposed above said level of the liquid and extending substantially across the tank and defining therebetween a slot for receiving developer from said fluid displacement means and defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment, whereby the crest of said standing wave presents uniform intimate contact with the surface of a sheet of copy paper passing thereover; and e. a tank sealing means adapted to automatically seal said slot to thereby completely cover said tank when the fluid displacement means is shut down.
 2. An apparatus as defined by claim 9, wherein said fluid displacement means includes rotating means having a contoured peripheral surface.
 3. An apparatus as defined by claim 2, wherein said rotating means includes a pair of axially parallel counterrotating gears journalled in opposite ones of said upstanding walls and extending radially below said liquid level, said gears being radially spaced from each other no closer than meshing engagement and no greater than would allow said gears to cooperatively create a suspended pool in the area thereabove and therebetween.
 4. An apparatus as defined by claim 3, wherein said gears are axially elongate spur gears.
 5. An apparatus as defined by claim 3, wherein the leading faces of the teeth on said gears form a concave surface, whereby said teeth raise larger amounts of developer from said liquid level to said receiving means.
 6. An apparatus as defined by claim 1, wherein said baffle means includes a first and second splash plate having first and second baffles extending respectivelY from inner edges thereof and spaced from each other to form a slot therebetween, said first and second baffles forming an angle with the plane of said first and second splash plates, respectively, of between about 25* and 35*, whereby said developer is raised by said fluid displacement means through said slot and into said baffle means.
 7. An apparatus as defined by claim 6, wherein the gap width of said slot is between about 28 and 32 percent of the pitch diameter of said gears.
 8. An apparatus as defined in claim 6, where the length of said first baffle is between about 10 and 15 percent of the pitch diameter of said gears and the length of said second baffle is between about 25 and 35 percent of said pitch diameter.
 9. An apparatus for wet development of latent electrostatic images, comprising: a. a tank having a bottom wall and upstanding walls to contain a liquid developer; b. fluid displacement means for raising said developer above the level of the liquid in said tank; c. actuating means for driving said fluid displacement means at a speed sufficient to create a standing wave of constantly replenished developer at a location generally above said rotating means; d. a rectangularly shaped first baffle suitably supported over said fluid displacement means and pitched therefrom toward said bottom wall; and e. a rectangularly shaped second baffle suitably supported over said fluid displacement means and pitched therefrom toward said bottom wall at an equal and opposite angle to the angle at which said first baffle is pitched, said second baffle being longer than said first baffle and being spaced from said first baffle to form a slot which is cooperatively associated with said fluid displacement means to receive said developer raised thereby, said first and second baffles defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment, whereby said fluid displacement means raises developer from said liquid level through said slot and the crest of said standing wave presents uniform intimate contact with the surface of a sheet of copy paper passing thereover.
 10. An apparatus for wet development of latent electrostatic images, comprising; a. a tank having a base and upstanding walls to contain a quantity of liquid developer; b. fluid displacement means for raising said developer above the level of the liquid in said tank; c. actuating means for driving said fluid displacement means to create a standing wave of constantly replenished developer at a location generally above said fluid displacement means; d. baffle means for receiving developer from said fluid displacement means and defining an area of containment for said standing wave and for stabilizing the surface characteristics of said standing wave within said area of containment; and e. tank sealing means for minimizing the evaporation of developer from said tank when said fluid displacement means are not in motion, said tank sealing means including; I. a cover cooperatively associated with said baffle means; and II. means for raising said cover above said baffle means when said fluid displacement means are being driven and for lowering said cover onto said baffle means when said fluid displacement means are not being driven; whereby said cover is lowered on top of said baffle means when said fluid displacement means are not being driven and said cover is raised when said fluid displacement means are being driven.
 11. An apparatus as defined by claim 10, which additionally includes a time delay control means for maintaining said tank sealing means in a raised position for a predetermined period of time subsequent to said fluid displacement means no longer being driven, whereby said developer has time to drain below said baffle means before said tank sealing means is lowered doWn onto said baffle means. 